The long-term goal of this research is to understand the changes that take place in the respiratory control system with aging in both males and females. The fundamental hypothesis guiding this proposal is that sex hormones have gender-specific effects on the respiratory control system that are affected by age. We propose that with increasing age, and an accompanying decline in sex hormone levels, there are gender- specific effects on the neural control of breathing. We propose that sex hormones are tightly linked to neurotransmitter function, specifically serotonin (5HT) and gamma aminobutyric acid (GABA), and thereby influence respiratory plasticity. These sexually dimorphic effects may underlie breathing disorders that differentially affect both young and old men and women. We will study two populations of respiratory motoneurons in a rat model: hypoglossal motoneurons innervating the tongue, and phrenic motoneurons innervating the diaphragm. These two motor neuron pools are differentially affected by sex hormones in males and females, and also by aging. Five specific aims are proposed. The first two specific aims assess whether sex hormone replacement therapy can be used to reverse the age-associated decline in respiratory plasticity that is seen in male and female rats, using an electrophysiological approach. The third specific aim is designed to assess whether ventilatory control in unanesthetized male and female rats is affected by the hormonal changes associated with aging, particularly the hypoxic and hypercapnic ventilatory responses, using barometric plethysmography. The fourth specific aim targets 5HT-containing neurons in the caudal raphe nuclei as an important site of action of sex hormones, and uses retrograde trans-synaptic viral labeling from the tongue, combined with immunoreactive markers for both 5HT and sex hormone receptors, to identify sex differences in these neurons. The fifth specific aim addresses an alternative or additional site of action of sex hormones: neurons containing GABA that synapse on respiratory motor neurons. We will use a combination of immunocytochemistry, immunoblot and RT-PCR to measure markers of GABA and the GABA-A receptor in the hypoglossal nuclei of male and female rats, and assess age-associated changes. These studies will lead to a greater understanding of breathing disorders that occur more frequently in older people, the best-recognized of which is obstructive sleep apnea (OSA). OSA and other breathing disorders are far more prevalent in men than in women, and the proposed studies will lead to a greater understanding of why such sex differences occur. Ultimately, these studies may enable us to identify age- and gender-appropriate pharmacological interventions for breathing disorders.